What are gills in biology, what role do they play? Why were they formed, what is their difference from the lungs that we breathe? Why do people have only one oxygen production system? These, as well as some other issues, will be considered.
Introductory information
Begin with a definition. What are gills? These are specific respiratory organs that specialize in conducting gas exchange between the body and the aquatic environment. The presence of active or passive circulation of water and blood or (hemo) lymph, as a rule, allows to ensure the maximum diffuse absorption of oxygen by constantly leaking blood, which is depleted for the content of O 2 . Gills can vary significantly in origin, structure, and location in different species and families. Let's take a closer look at this.
What are they?
It was considered what gills are in the general case. And now more specialized:
- Gills in invertebrates. Most often, they are decorated in the form of outgrowths of the body walls. The simplest samples are usually located in the head of tubular polychaetes and on the extremities of free-living. Although, in mollusks they are represented by densely coated lateral cilia, due to which the flow of water is mainly created. In crustaceans, gills are formed in the form of abdominal extremities or chest outgrowths. Although, small representatives of this family do not have anything like this, they breathe the surface of the whole body. Gills are also used by some insects. But more on that later.
- Gills in lower aquatic vertebrates. Often found in the form of symmetrical paired slits in the pharynx, which are washed by water. On the back and front surface of this place there are lamellar outgrowths. They are divided into gill petals of ectodermal origin. It is in them that gas exchange is carried out. Although, in myxins and lampreys, this takes place in paired gill sacs, which have a lenticular shape and also endodermal origin. But this is far from all. In vertebrates, the gill apparatus has a skeleton. It is represented by cartilage and bone arches.
- Amphibians in the larval stage. They have branched external gills. But with the formation of the lungs, they disappear. Only at sirens and proteas they remain until the end of life. Although, some amphibians can simultaneously have both breathing systems.
That's what gills are.
Say a word about individual representatives
The best known to the general public is the presence of such respiratory organs in fish and similar aquatic inhabitants. This is due to the fact that almost everyone on one or another regular basis uses them. Gas exchange is carried out either during movement, or during rest. There are also representatives with self-opening holes. In bone fish, water is pumped through the gills due to the action of the caps and mouth, even when stationary.
But the most unusual is the gills of insects. They often have a closed tracheal system, that is, there is no direct communication with the external environment. To ensure the ability to breathe in the evolutionary process, a special adaptation was developed. It is called tracheal gills. It usually looks like a fold of skin with a leaf-shaped shape. They are usually located on the abdomen near the location of the spiracles.
But what about people?
It has already been considered what gills are in fish, insects and vertebrates. That's just about people there was still no talk. Once upon a time, our ancestors also had a gill, even when they lived in the aquatic environment. But then the development of sushi began. As a result, the gills were replaced with more efficient lungs, which can quickly extract much-needed oxygen from the air. And all ineffective organs atrophied from uselessness.